PROJECT SUMMARY/ABSTRACT Continuously elevated parathyroid hormone (PTH) causes hypercalcemia/hypophosphatemia and enhances bone turnover. This results in bone loss because bone resorption increases more prominently than bone formation. In contrast, daily PTH injection enhances bone formation with only transient mineral ion changes. Surprisingly, mice expressing a constitutively active PTH/PTHrP receptor (PTHR1) in bone (Col1-H223R or Dmp1-H223R) show profound bone mass increases. Likewise, patients affected by pseudohypoparathyroidism (PTH-resistance in kidney, but not in bone), treated with calcium and 1,25(OH)2 vitamin D (1,25D), can show a major increase in bone formation, despite elevated PTH levels. Findings in transgenic mice and a human disease thus suggested that persistent PTHR1 activation can increase bone mass, if excess urinary phosphate excretion is prevented. Considerable evidence indicates that urinary phosphate excretion by PTH depends not only on cAMP/PKA signaling, but also on IP3/PKC-dependent mechanisms. In fact, D/D mice expressing a phospholipase C (PLC)-deficient PTHR1 do not sustain phosphate excretion when the PTHR1 is continuously activated. Furthermore, wild-type animals receiving long-term infusions of [Trp1]PTH(1-34), a biased PTH analog with impaired IP3 signaling, failed to sustain phosphate excretion and 1,25D production. In addition to its renal importance, PTH-stimulated PLC/IP3/PKC signaling is required for bone formation. For example, tibiae of wild-type mice undergoing prolonged PTH-dependent PTHR1 activation showed rapid expansion of fibroblast-like stromal bone (FSB) cells; such osteoblast precursors were not observed in D/D mice with continuous PTH elevations and in wild-type mice infused with [Trp1]PTH(1-34), thus supporting the conclusion that PLC-signaling at the PTHR1 is required for normal bone formation. We will now determine whether persistent IP3/PKC-signaling at the PTHR1 is required for long-term regulation of phosphate homeostasis and 1,25D production, and whether continuous PTHR1 activation can enhance bone formation, if excess urinary phosphate excretion can be prevented. Two aims will be pursued: Aim 1 will explore further whether PTH-dependent IP3/PKC signaling is required for sustained phosphate excretion, for 1,25D production, and for expansion of osteoblast precursors. Aim 2 will determine whether the high bone mass in Col1-H223R mice and animals with osteocyte-specific SIK2/3 ablation can be reversed by promoting renal phosphate excretion through PTH-independent interventions, namely through a novel NPT2a inhibitor or through recombinant FGF23. We will also determine whether ?bone-seeking? PTH analogs can induce bone formation because of continuous local PTHR1 activation, yet limited renal actions, i.e. no phosphaturia and no 1,25D increase. Our investigations will help clarify further the role of PLC-dependent PTHR1 signaling in kidney and bone, and the importance of phosphate in maintaining high bone mass in two genetically altered mice.